1. Field of the Invention
This invention relates to the tracking of assets, including goods and vehicles, using the Global Positioning System (GPS), and more particularly to a protocol and mechanism for centralized asset tracking communications.
2. Background Description
Goods shipped from a manufacturing plant, warehouse or port of entry to a destination are normally tracked to assure their timely and safe delivery. Tracking has heretofore been accomplished in part by use of various shipping documents and negotiable instruments, some of which travel with the goods and others of which are transmitted by post or courier to a receiving destination. This paper tracking provides, a record which is completed only on the safe delivery and acceptance of the goods. However, there sometimes is a need to know the location of the goods. Knowledge of the location of goods can be used for inventory control, scheduling and monitoring.
Shippers have provided information on the location of goods by tracking their vehicles, knowing what goods are loaded on those vehicles. Goods are often loaded aboard shipping containers or container trucks, for example, which are in turn loaded aboard railcars. Various devices have been used to hack such vehicles. In the case of railcars, passive radio frequency (RF) transponders mounted on the cars have been used to facilitate interrogation of each car as it passes a way station and supply the car's identification. This information is then transmitted by a radiated signal or land line to a central station which tracks the locations of cars. This technique, however is deficient in that whenever a particular railcar remains on a siding for an extended period of time, it does not pass a way station. Moreover, way station installations are expensive, requiting a compromise that results in way stations being installed at varying distances, depending on the track layout. Thus, the precision of location information varies from place to place on the railroad.
Recently, mobile tracking units have been used for tracking various types of vehicles, such as trains. Communication has been provided by means of cellular mobile telephone or RF radio link. Such mobile tracking units are generally installed aboard the locomotive which provides a ready source of power. However, in the case of shipping containers, container truck trailers and railcars, a similar source of power is not readily available. Mobile tracking units which might be attached to containers and vehicles must be power efficient in order to provide reliable and economical operation. Typically, a mobile tracking unit includes a navigation set, such as a Global Positioning System (GPS) receiver or other suitable navigation set, responsive to navigation signals transmitted by a set of navigation stations which may be either space-based or earth-based. In each case, the navigation set is capable of providing data indicative of the vehicle location based on the navigation signals. In addition, the tracking unit may include a suitable electromagnetic emitter for transmitting to a remote location the vehicle's location data and other data acquired from sensing elements on board the vehicle. Current methods of asset localization require that each item tracked be individually equipped with hardware which determines and reports location to a central station. In this way, a tracked asset is completely "ignorant" of other assets being shipped or their possible relation to itself. In reporting to the central station, such system requires a bandwidth which scales approximately with the number of assets being reported. The aggregate power consumption over an entire such system also scales with the number of assets tracked. Further, since both the navigation set and the emitter are devices which, when energized, generally require a large portion of the overall electrical power consumed by the mobile tracking unit, it is desirable to control the respective rates at which such devices are respectively activated and limit their respective duty cycles so as to minimize the overall power consumption of the mobile tracking unit.
Most present-day asset tracking systems are land-based systems wherein a radio unit on the asset transmits information to wayside stations of a fixed network, such as the public land mobile radio network or a cellular network. These networks do not have ubiquitous coverage, and the asset tracking units are expensive. A satellite-based truck tracking system developed by Qualcomm Inc., kuown as OMNITRACS, is in operation in the United States and Canada. This system requires a specialized directional antenna and considerable power for operation, while vehicle location, derived from two satellites, is obtained to an accuracy of about one-fourth kilometer. U.S. Pat. No. 5,129,605 to Burns et al. describes a rail vehicle positioning system for installation on the locomotive of a train and which uses, to provide input signals for generating a location report, a GPS receiver, a wheel tachometer, transponders, and manual inputs from the locomotive engineer.
In an asset tracking system disclosed in U.S. application Ser. No. 08/484,950, entitled "Local Communication Network for Power Reduction and Enhanced Reliability in a Multiple Node Tracking System" by Welles et al. and in U.S. application Ser. No. 08/487,272 U.S. Pat. No. 5,588,505 entitled "Protocol and Mechanism for Primary and Mutter Mode Communication for Asset Tracking" by Ali et al. assigned to the instant assignee and incorporated herein by reference, a tracking system based on a "mutter" mode local area network is used to generate dam which are transmitted to a central station. In this asset tracking system, there are two modes of communication. One mode is communication between the central station and the tracking units, which is usually via satellite: The second mode is a local area network, referred to as the "mutter" mode, between tracking units. One of the tracking units, denoted the master unit, communicates with the central station.
One of the chief challenges in using the first mode of communication is to devise a protocol for the communications that will provide efficient use of the communication facilities and respect the special sensitivities of the reporting scenario. Such protocol should meet the following guidelines:
1. The protocol should be two-way, thereby supporting transmission to and from a central station. PA0 2. The protocol must accommodate a large number of assets and be scalable so that assets can be added and deleted without impacting normal service. PA0 3. The protocol must accommodate variable length messages. The variable length may arise from a number of considerations; for example, the individual asset may have extra sensor data to report in addition to its location. PA0 4. The protocol must have a chatter suppression feature to allow selective turn-off of a specific malfunctioning asset's transmitter. PA0 5. The protocol must function efficiently if used over an extremely long path such as is implied by use of a geostationary satellite. PA0 6. The protocol must allow encryption or a privacy feature to be added later without significantly impacting the capacity. PA0 7. The protocol must be sufficiently robust to allow an asset to enter the system at any time without knowledge that cannot be gleaned following its entry into the system, and must tolerate occasional transmission errors and not be unstable but degrade gracefully under additional load. PA0 8. The protocol must not require the assets to be receiving all the time but accommodate a duty cycle significantly less than 100% for periods of monitoring communication frequencies.
The protocol must be designed to be easily adjusted and nominally reprogrammable to allow presentation of its efficiency as the operational scenario matures.